rfc759.txt

RFC 相关的技术文档

IEN: 113
RFC: 759
                                    
                                    
                                    
                                    
                                    
                                    
                       INTERNET MESSAGE PROTOCOL
                                    
                                    
                                    
                           Jonathan B. Postel

















                              August 1980
                                    
                                    
                                    
                                    
                     Information Sciences Institute
                   University of Southern California
                           4676 Admiralty Way
                   Marina del Rey, California  90291

                             [phone]







August 1980                                                             
                                               Internet Message Protocol



                           TABLE OF CONTENTS

    PREFACE ........................................................ iii

1.  INTRODUCTION ..................................................... 1

  1.1.  Motivation ................................................... 1
  1.2.  Scope ........................................................ 1
  1.3.  The Internetwork Environment ................................. 2
  1.4.  Model of Operation ........................................... 2
  1.5.  Interfaces ................................................... 4

2.  FUNCTIONAL DESCRIPTION ........................................... 5

  2.1.  Terminology .................................................. 5
  2.2.  Assumptions  ................................................. 5
  2.3.  General Specification ........................................ 6
  2.4.  Mechanisms ................................................... 7
  2.5.  Relation to Other Protocols ................................. 10

3.  DETAILED SPECIFICATION .......................................... 13

  3.1.  Overview of Message Structure ............................... 13
  3.2.  Message Structure ........................................... 14
  3.3.  Identification .............................................. 15
  3.4.  Command ..................................................... 15
  3.5.  Document .................................................... 19
  3.6.  Message Objects ............................................. 20
  3.7.  Data Elements ............................................... 27

4.  OTHER ISSUES .................................................... 35

  4.1.  Accounting and Billing ...................................... 35
  4.2.  Addressing and Routing ...................................... 36
  4.3.  Encryption .................................................. 37

5.  The MPM:  A Possible Architecture ............................... 39

  5.1.  Interfaces .................................................. 39
  5.2.  MPM Organization ............................................ 40

6.  EXAMPLES & SCENARIOS ............................................ 45

  Example 1:  Message Format ........................................ 45
  Example 2:  Delivery and Acknowledgment ........................... 47





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Internet Message Protocol
Table Of Contents



7.  SPECIFICATION SUMMARY ........................................... 55

  7.1.  Message Fields .............................................. 55
  7.2.  Deliver Message ............................................. 58
  7.3.  Acknowledge Message ......................................... 59
  7.4.  Probe Message ............................................... 61
  7.5.  Response Message ............................................ 62
  7.6.  Cancel Message .............................................. 64
  7.7.  Canceled Message ............................................ 66
  7.8.  Data Element Summary ........................................ 68

REFERENCES .......................................................... 69

  



































[Page ii]                                                         Postel


August 1980                                                             
                                               Internet Message Protocol



                                PREFACE



This is the second edition of this specification and should be treated
as a request for comments, advice, and suggestions.  A great deal of
prior work has been done on computer aided message systems and some of
this is listed in the reference section.  This specification was shaped
by many discussions with members of the ARPA research community, and
others interested in the development of computer aided message systems.
This document was prepared as part of the ARPA sponsored Internetwork
Concepts Research Project at ISI, with the assistance of Greg Finn,
Suzanne Sluizer, Alan Katz, Paul Mockapetris, and Linda Sato.

                                                              Jon Postel



































Postel                                                        [Page iii]




IEN: 113                                                       J. Postel
RFC: 759                                                         USC-ISI
                                                             August 1980




                       INTERNET MESSAGE PROTOCOL



                            1.  INTRODUCTION

This document describes an internetwork message system.  The system is
designed to transmit messages between message processing modules
according to formats and procedures specified in this document.  The
message processing modules are processes in host computers.  Message
processing modules are located in different networks and together
constitute an internetwork message delivery system.

This document is intended to provide all the information necessary to
implement a compatible cooperating module of this internetwork message
delivery system.

1.1.  Motivation

  As computer supported message processing activities grow on individual
  host computers and in networks of computers, there is a natural desire
  to provide for the interconnection and interworking of such systems.
  This specification describes the formats and procedures of a general
  purpose internetwork message system, which can be used as a standard
  for the interconnection of individual message systems, or as a message
  delivery system in its own right.

  This system also provides for the communication of data items beyond
  the scope of contemporary message systems.  Messages can include data
  objects which could represent drawings, or facsimile images, or
  digitized speech.  One can imagine message stations equipped with
  speakers and microphones (or telephone hand sets) where the body of a
  message or a portion of it is recorded digitized speech.  The output
  terminal could include a graphics display, and the message might
  present a drawing on the display, and verbally (via the speaker)
  describe certain features of the drawing.  This specification provides
  for the composition of complex data objects and their encoding in
  machine independent basic data elements.

1.2.  Scope

  The Internet Message Protocol is intended to be used for the
  transmission of messages between networks.  It may also be used for
  the local message system of a network or host.  This specification was



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Internet Message Protocol
Introduction



  developed in the context of the ARPA work on the interconnection of
  networks, but it is thought that it has a more general scope.

  The focus here is on the internal mechanisms to transmit messages,
  rather than the external interface to users.  It is assumed that a
  number of user interface programs will exist.  These will be both new
  programs designed to work with this system and old programs designed
  to work with earlier systems.

1.3.  The Internetwork Environment

  The internetwork message environment consists of processes which run
  in hosts which are connected to networks which are interconnected by
  gateways.  Each network consists of many different hosts.  The
  networks are tied together through gateways.  The gateways are
  essentially hosts on two (or more) networks and are not assumed to
  have much storage capacity or to "know" which hosts are on the
  networks to which they are attached [1,2].

1.4.  Model of Operation

  This protocol is implemented in a process called a Message Processing
  Module or MPM.  The MPMs exchange messages by establishing full duplex
  communication and sending the messages in a fixed format described in
  this document.  The MPM may also communicate other information by
  means of commands described here.

  A message is formed by a user interacting with a User Interface
  Program or UIP.  The user may utilize several commands to create
  various fields of the message and may invoke an editor program to
  correct or format some or all of the message.  Once the user is
  satisfied with the message it is submitted for transmission by placing
  it in a data structure read by the MPM.

  The MPM discovers the unprocessed input data (either by a specific
  request or by a general background search), examines it, and, using
  routing tables (or some other method), determines which outgoing link
  to use.  The destination may be another user on the same host, one on
  another host on a network in common with the same host, or a user in
  another network.

  In the first case, another user on this host, the MPM places the
  message in a data structure read by the destination user, where that
  user's UIP will look for incoming messages.

  In the second case, the user on another host in this network, the MPM
  transmits the message to the MPM on that host.  That MPM then repeats


[Page 2]                                                          Postel


August 1980                                                             
                                               Internet Message Protocol
                                                            Introduction



  the routing decision, and discovering the destination is local to it,
  places the message in the data structure shared with the destination
  user.

  In the third case, the user on a host in another network, the MPM
  transmits the messages to an MPM in that network if it knows how to
  establish a connection directly to it; otherwise, the MPM transmits
  the message to an MPM that is "closer" to the destination.  An MPM
  might not know of direct connections to MPMs in all other networks,
  but it must be able to select a next MPM to handle the message for
  each possible destination network.

  An MPM might know a way to establish direct connections to each of a
  few MPMs in other nearby networks, and send all other messages to a
  particular big brother MPM that has a wider knowledge of the internet
  environment.

  An individual network's message system may be quite different from the
  internet message system.  In this case, intranet messages will be
  delivered using the network's own message system.  If a message is
  addressed outside the network, it is given to an MPM which then sends
  it through the appropriate gateways to (or towards) the MPM in the
  destination network.  Eventually, the message gets to an MPM on the
  network of the recipient of the message.  The message is then sent via
  the local message system to that host.

  When local message protocols are used, special conversion programs are
  required to transform local messages to internet format when they are
  going out, and to transform internet messages to local format when
  they come into the local environment.  Such transformations
  potentially lead to information loss.  The internet message format
  attempts to provide features to capture all the information any local
  message system might use.  However, a particular local message system
  is unlikely to have features equivalent to all the possible features
  of the internet message system.  Thus, in some cases the
  transformation of an internet message to a local message discards some
  of the information.  For example, if an internet message carrying
  mixed text and speech data in the body is to be delivered in a local
  system which only carries text, the speech data may be replaced by the
  text string "There was some speech here".  Such discarding of
  information is to be avoided when at all possible, and to be deferred
  as long as possible; still, the possibility remains that in some cases
  it is the only reasonable thing to do.






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                                                             August 1980
Internet Message Protocol
Introduction



1.5.  Interfaces

  The MPM calls on a reliable communication procedure to communicate
  with other MPMs.  This is a Transport Level protocol such as the
  Transmission Control Protocol (TCP) [3].  The interface to such a
  procedure conventionally provides calls to open and close connections,
  send and receive data on a connection, and some means to signal and be
  notified of special conditions (i.e., interrupts).

  The MPM receives input and produces output through data structures